MCAT Biology and Biochemistry: 6 High-Yield Practice Questions
The Biological and Biochemical Foundations section of the MCAT (BBFL) tests molecular biology, biochemistry, and physiology in the context of passages and standalone questions. These 6 questions cover the highest-tested patterns.
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High-Yield Topics Covered
Enzyme kinetics (Km, Vmax, inhibition types)
DNA replication and repair
Protein structure and folding
Glycolysis and citric acid cycle
Cell signaling pathways
Membrane transport
Question 1 of 6Score: 0 / 0
Q1Enzyme Kinetics
An enzyme has a Km of 2 mM and Vmax of 100 μmol/min. A competitive inhibitor is added at constant concentration. How do Km and Vmax change?
Explanation: Competitive inhibition: the inhibitor competes with substrate for the active site and can be overcome by increasing substrate concentration. Effect: apparent Km INCREASES (enzyme appears to have lower affinity for substrate), Vmax is UNCHANGED (sufficient substrate can still achieve maximum velocity). Noncompetitive inhibitor: Km unchanged, Vmax decreases.
Q2DNA Replication
During DNA replication, the enzyme responsible for synthesizing a new DNA strand from 3' to 5' on the template is:
Explanation: DNA polymerase reads the template strand 3'→5' and synthesizes the new strand 5'→3'. This means the enzyme travels along the template in the 3'→5' direction to add nucleotides in the 5'→3' direction. Primase makes the RNA primer; helicase unwinds the helix; ligase seals Okazaki fragments.
Q3Protein Structure and Enzyme Regulation
A protein is described as having four subunits, each with its own active site. Binding of substrate to one subunit increases substrate affinity at the other subunits. This describes:
Explanation: Positive cooperativity: binding of substrate to one subunit of a multimeric enzyme causes conformational change that increases substrate affinity of remaining subunits. Classic example: hemoglobin binding oxygen (though hemoglobin is not an enzyme). Sigmoidal (not hyperbolic) kinetics on substrate-velocity curve.
Q4Cell Signaling
A cell signaling pathway involves a ligand that binds a receptor, activates adenylyl cyclase, increases cAMP, and activates PKA. This is a:
Explanation: Gs-coupled GPCR → activates adenylyl cyclase → increases cAMP → activates PKA (protein kinase A). Examples: epinephrine (β-adrenergic), glucagon, PTH. Gi-coupled GPCRs inhibit adenylyl cyclase. RTK pathways activate RAS/MAP kinase. JAK-STAT is used by cytokines. SMAD is TGF-β.
Q5Metabolism and Bioenergetics
During high-intensity anaerobic exercise, pyruvate is converted to lactate. The primary purpose of this reaction is:
Explanation: Lactate dehydrogenase converts pyruvate to lactate while oxidizing NADH → NAD+. The regenerated NAD+ is essential for continued glycolysis (which requires NAD+ as an electron acceptor in the glyceraldehyde-3-phosphate dehydrogenase step). Without this recycling, glycolysis halts. Lactate is later converted back to glucose (Cori cycle) in the liver.
Q6Membrane Transport and Physiology
A mutation in the Na+/K+ ATPase causes it to become permanently inactive in nerve cells. The immediate consequence would be:
Explanation: The Na+/K+ ATPase maintains resting membrane potential by pumping 3 Na+ out and 2 K+ in. If inactivated, Na+ accumulates intracellularly and K+ is lost → membrane depolarizes toward 0 mV. The neuron loses the ability to maintain a resting potential and cannot generate normal action potentials.
The Biological and Biochemical Foundations (BBFL) section is 59 questions and comprises about 25% of the MCAT composite score. Within BBFL, approximately 25% is biochemistry, 65% is biology, and 10% is general chemistry. The MCAT emphasizes biochemistry heavily — enzyme kinetics, amino acid properties, metabolism, and molecular biology are all routinely tested.
What is the difference between Km and Vmax?
Km (Michaelis constant) = substrate concentration at which reaction velocity = ½ Vmax. It is an inverse measure of enzyme affinity — lower Km = higher affinity (enzyme reaches half-max velocity at low substrate). Vmax = maximum reaction velocity when enzyme is saturated with substrate. Competitive inhibitors increase apparent Km (lower affinity) without changing Vmax. Noncompetitive inhibitors decrease Vmax without changing Km.
What metabolism pathways are highest yield for MCAT?
Highest-yield metabolic pathways: glycolysis (10 steps, ATP yield, key enzymes), pyruvate dehydrogenase complex (acetyl-CoA production), citric acid cycle (NADH/FADH2 production, key intermediates), oxidative phosphorylation (ATP synthase, electron transport chain), and the Cori cycle (lactate → liver → glucose). Also high-yield: fatty acid oxidation (beta-oxidation), ketogenesis, and gluconeogenesis (key irreversible steps).
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